Urea is generally produced from ammonia and carbon dioxide. It can be prepared by introducing an ammonia excess together with carbon dioxide at a pressure between 12 and 40 MPa and at a temperature between 150° C. and 250° C. into a urea synthesis zone. The resulting urea formation can be presented best in the form of two consecutive reaction steps, in the first step ammonium carbamate being formed according to the exothermic reaction:2NH3+CO2→H2N—CO—ONH4 after which the ammonium carbamate formed is dehydrated in the second step to give urea according to the endothermic equilibrium reaction:H2N—CO—ONH4↔H2N—CO—NH2+H2O
The extent to which these reactions take place depends among other things on the temperature and the ammonia excess used. The reaction product obtained in a urea synthesis solution substantially consists of urea, water, unbound ammonia and ammonium carbamate. The ammonium carbamate and the ammonia are removed from the solution and are generally returned to the urea synthesis zone.
In addition to the above-mentioned solution in the urea synthesis zone, a gas mixture is formed which consists of unconverted ammonia and carbon dioxide together with inert gases, the so called reactor off-gas. The urea synthesis section may comprise separate zones for the formation of ammonium carbamate and urea. These zones may also be combined in a single apparatus.
Different urea production process exist. These processes, and by analogy the plants in which these processes are conducted, generally provide for the following stages: synthesis, recovery of unreacted starting materials, downstream processing, and finishing. Thereby synthesis and recovery sections are applied that are connected with each other so as to form a synthesis loop, whereby starting materials (ammonia and carbon dioxide, particularly in the form of ammonium carbamate) are recovered and recycled back to synthesis stage. The output of the synthesis loop is generally a purified aqueous urea stream, having a concentration of 50 wt. % urea or higher, generally up to 75-80 wt % before said stream is subjected to final concentration step(s).
The downstream processing generally refers to one or more sections, zones, or units in which the aforementioned aqueous urea stream is further concentrated. Such further concentration is typically conducted by evaporation, and the concentration section is frequently referred to as an evaporation section.
One of the challenges in urea production concerns controlling the amount of biuret formed as a by-product, and generally present in urea products such as prills or granules. Biuret is dimer of urea, and is formed under release of ammonia. The amount of biuret is an indicator of the urea quality as can be sold. Typically, a worldwide standard specification for biuret in urea products, is below 1 wt. %. E.g., for fertilizer purposes, the amount of biuret is generally below 0.9 wt. %. For other applications, such as the use of an aqueous urea solution in a unit for the reduction of NOx in diesel exhaust gases (particularly known as Diesel Exhaust Fluid, traded as AdBlue®), the biuret content is required to be still lower.
In urea plants operating on the basis of old, once-through technology the formation of biuret is not a significant problem. Modern plants, such as urea stripping plants, however tend to result in a higher amount of biuret formed. It remains desired to better control biuret production.
An additional problem is that it is more difficult to produce urea according to desired biuret specifications, in the event that the plant in which the urea is produced, is not operated on full capacity. Generally, biuret levels are guaranteed for a plant operating at full capacity. In practice, this means that manufacturers operating their plants at reduced capacity, run a risk that the products produced do not meet specifications for all end-uses. It would be desired to provide a urea manufacturing process, and a plant suitable for such process, that allows controlling biuret formation also in the event that the plant in which the urea is produced is operated at a reduced capacity.
GB959.358 discloses a process for producing urea prills of low biuret content according to which urea containing degasified reactor effluent is passed from the primary purification zone to a second purification zone wherein the effluent is heated under specified conditions of temperature and pressure. A highly concentrated biuret-containing melt of urea is withdrawn from the second purification zone. The biuret concentration of the urea withdrawn from that second purification zone is said to depend on the degree of concentration of the urea achieved. The biuret concentration is further reduced by contacting the urea melt with an ammonia containing gas at a temperature above the melting point of pure urea for a period of time sufficient to achieve equilibrium between the reacting ammonia and biuret and urea.
U.S. Pat. No. 3,223,145 discloses a method for preparing urea prills which are at the same time mechanically strong and dense, and have a low tendency to pick up water, buy controlling the overall urea heating time and temperature. As a feed, use is made of molten urea with a low water content and an excessive biuret content. Biuret contained in the molten urea is removed by carrying out dehydration of the molten urea as rapidly as possible, at as low a temperature as possible, or in other words by minimizing the time during which and the temperature to which dehydrated, molten urea is heated.